This invention concerns the deasphalting of residual hydrocarbon oils, such as oil distillation residues, residues from shale oils or bituminous shales or heavy products from destructive distillation or from coal hydrogenation.
Residual hydrocarbon oils, which contain asphaltic materials, cannot be directly subjected to such treatments as hydrodesulfurization, hydrocracking or catalytic cracking in view of their excessive asphalt and metals content.
It has thus been proposed to subject them to a prior deasphalting treatment with a solvent selected from the saturated or olefinic aliphatic hydrocarbons of low molecular weight.
It is known that the yield of deasphalted oil is higher when the hydrocarbon used as solvent has a greater number of carbon atoms, with however two disadvantages: first, this increased yield is obtained with a simultaneous reduction of the oil purity, particularly its metal content; and second, the heavier the solvent, the harder and the less fusible the resultant asphalt.
Consequently, for practical reasons and although the use of hydrocarbons with 3 to 7 carbon atoms has been proposed, most of the known plants operate with C.sub.3, C.sub.4, mixtures of C.sub.3 +C.sub.4 and, in a reduced number of cases, C.sub.5 hydrocarbons.
A difficulty to overcome is the treatment of the asphaltic phase rejected in the course of the deasphalting. This phase comprises a substantial proportion of extraction solvent and must be separated from the latter. This is commonly obtained by vaporizing the solvent and/or stripping with an inert gas such as nitrogen or steam; the vaporization of the solvent requires supplying an important amount of heat to the asphaltic phase.
In the known plants, the asphaltic phase is passed through a furnace heated with a flame (U.S. Pat. Nos. 2,943,050; 3,423,308 and 4,017,383) although steam can be used for propane and some light mixtures of propane with butane (U.S. Pat. No. 3,627,675).
It is clear that the heavier the solvent, and the higher the temperature of the furnace, although the temperature is limited by the tendency of the solvent to decompose when contacted with the walls of the furnace. Asphalt is considered to decompose at from 310.degree. to 330.degree. C. However, it is difficult to control the temperature of the furnace walls. Not only is the temperature not the same in all parts of the furnace, but also the optimum temperature varies in the course of time, depending on the nature, itself variable, of the feed charge to reheat.
The decomposition of the asphalt results in deposition of scale which interferes with good heat transmission and requires further heating which further increases the number of hot points; the clogging of the ducts can even necessitate stopping the operation.
This disadvantage occurs even with butane, but it is particularly important when using a hydrocarbon with 5 to 7 carbon atoms, inasmuch as the resultant asphalt is more and more viscous.
The object of the present invention is to describe a deasphalting (demetallization) process which obviates the above drawbacks and thus allows the treatment, without fouling difficulty and over long periods, of residual hydrocarbon oils by means of hydrocarbons having from 4 to 7 carbon atoms, for example, isobutane, n-butane, neo-pentane, n-pentane, isohexane or C.sub.4, C.sub.5 and C.sub.6 cuts.